FastCal

1 DAS CONVERTER BOARD IDENTIFICATION CHECK

Additional calibration scans are needed to adjust for gain changes. After these scans are completed you must redo FastCal. After you have read this press CONTINUE.

If converter board check fails, a DAS Gain Calibration is required.

2 DIRTY MYLAR WINDOW SCAN

The first scan taken should check to see if there is any contrast or other material on the Mylar window that will corrupt the calibration scans. Four one-second rotating scans, no tracking at this time, should be taken. The scan techniques are to be 80kvp, 20 ma, aperture 4 x 125, 4 x 250, 4 x 375, and 4 x500 respectively. If the 20 point filtered offset corrected channel 762 data vs views divided by the offset corrected view averaged value of channel 762 falls below .90 for any scan, for any row, a message must be displayed to the user and a response from the user is needed before continuing. The user should be allowed to go ahead without further action, or clean the Mylar window and repeat the blockage scan. The message should say:

Please check Mylar window and clean if necessary to assure proper scanner operation. Indicate if you want to repeat the check scan or continue with the FastCal.

Send message to GE sys log.

If continuing without cleaning the Mylar window, the calibration vectors may cause image artifacts.

3 Tube Warm up scans

4 CHECK TIME OF LAST COLLIMATOR CALIBRATIONS

Check the last time Collimator Calibration was performed. If Collimator Cal was performed within 24 hours, it is not necessary to update the Collimator Cal parameters, and the system can skip all steps pertaining to Collimator Cal.

5 COLLIMATOR CALIBRATION (SWEEP SCAN)

Before the first standard FastCal scan is performed, but after tube warms up, a sweep scan is taken and a Collimator Calibration is performed for that technique. There are eight sweep scans-one for each aperture and focal spot size combination. One sweep scan is performed for every FastCal executed, and therefore the entire set of Collimator Cals refresh after eight FastCals are performed. Also, measure mode calculations are made, although only the results for large spot with 4x125 or 4x500 apertures are used.

The new Collimator Cal is compared to the old Collimator Cal in the following way:

1. Consider the range of the ratios for the old data and pick three ratios: the two ratios 10% from either end of the range, and the ratio in the middle.

2. Evaluate the new data at these three ratios, and compare to the values obtained with the old data.

6 BLOCKED CHANNEL CALCULATION

During the FastCal scan, the offset corrected signals are view averaged for the inside rows (1A and 1B) for channel 762. These averaged signals are then normalized with respect to the mAs per view and the DAS Gain, and multiplied by a threshold value referred to as the “blocked channel threshold”. During regular scanning, the normalized signals for each view are compared to the values obtained from the FastCal scan. If the value during the patient scan is lower than the value computed during the FastCal scan, it is assumed that the corresponding row in channel 762 is blocked for the view, and tracking is put on hold.

7 FASTCAL SCAN

During the FastCal scans, tracking takes place. However, there is no checking for blockage of z channel. Since the FastCal procedure checks for beam obstruction, there should be no blockage. The DCB computes the focal spot position.

8 DAILY IQ CHECK

FastCal also performs Daily IQ Check, which compares the center 30 channels of today’s FastCal vectors to yesterday’s. This is done to determine whether there is any significant change that could lead to an image artifact. If the limit check fails, a message is posted to the log and to a pop-up box on the screen. The database is updated regardless of whether the check passes or fails.

The following are the two circumstances that will cause the failure:

1. A hardware change, either after changing the detector or changing the center four DAS Converter cards will cause a failure message on the next FastCal.

   A hardware change will cause a significant change in the calibration vectors and trip the limit check. In this case, the error message on the first FastCal after the change can generally be ignored, provided the images look good.

2. A real change in the gain of the center channels, which could lead to an image artifact.

   The possible causes are contamination on the copper filter, tube port or bowtie filter or DAS.

   Refer to the Troubleshooting tab for Diagnostics and Image Quality tools to use for troubleshooting.